HIGH AVERAGE POWER OPERATION OF A SCRAPER-OUTCOUPLED FREE- ELECTRON LASER M. Shinn*, C. Behre, S. Benson, M. Bevins, D. Bullard, J. Coleman, L. Dillon-Townes, T. Elliott, J. Gubeli, D. Hardy, K. Jordan, R. Lassiter, G. Neil, S. Zhang TJNAF, Newport News, VA 23606 Abstract We describe the design, construction, and operation of a high average power free-electron laser using scraper outcoupling. Using the FEL in this all-reflective configuration, we achieved approximately 2 kW of stable output at 10 um. Measurements of gain, loss, and output mode will be compared with our models. INTRODUCTION Compared to other high average power laser systems, an advantage of a free-electron laser (FEL) is there are no heat dissipation issues within the gain medium. However, when operated in an oscillator configuration, one must mitigate and manage the effects of absorbed photons and the resultant thermally induced distortion of the otherwise spherical mirror surface. Previous analysis [1] and experiment [2] have shown that this distortion, when sufficiently large, causes the output to saturate. The obvious solution, to obtain low loss coatings and substrates, isnt always possible at certain wavelength ranges. For example, at 10.6 µm, the best outcoupler (OC) designs still have absorption levels in excess of 0.1% [3], which limits the extracted power to less than 10 kW [4]. In other wavelength regions such as the deep UV (< 150 nm) suitable substrates for outcoupling the output dont exist. In these cases, some other scheme for outcoupling the laser radiation from the resonator must be used. In FELs, hole outcoupling is traditional [5], but is not particularly efficient. For many high average power lasers it is customary to use a scraper mirror intracavity, i.e., an annular mirror were some fraction of the center of the laser mode is transmitted, and the wings of the mode are reflected out of the cavity. [6] Usually scraper outcoupling is used in positive branch unstable resonators, where the gain is very high and the outcoupling fraction may be large, e.g., > 30%. The gain in the majority of FELs is not so high, and the wiggler in the FEL requires a negative branch unstable resonator, which adds its own complications. We chose to use our existing near- concentric resonator and place the scraper mirror close to one of the end mirrors. Only one theoretical paper [7] has been published on the use of an annular scraper, but to our knowledge there are no experimental results. Therefore, we analyzed this cavity configuration to confirm that we would get efficient and stable output. Finding that it was stable, we then proceeded to build and test it. DESIGN AND IMPLEMENTATION The optical cavity configuration we modeled is shown in Fig, 1, and the resonator parameters are shown in Table 1. We chose to operate at ~ 10 µm, as we had been commissioning the IR Upgrade FEL at this wavelength using a conventional, transmissive outcoupling optical cavity, and had mirrors readily available. HR Wiggler Scraper HR Figure 1 Schematic layout of the FEL optical cavity with an annular scraper. The solid arrows denote the outcoupled beam, the dashed arrows denote the diffracted beam that strikes the rear of the scraper. Table 1 FEL resonator parameters Parameter Value Cavity length 32.042 m Rayleigh range 3 m Mirror radius of curvature 16.6 m Scraper position 15.4 m Scraper outcoupling 10% Given the single pass gain of our optical klystron was ~ 30%, and the extraction efficiency is highest when the outcoupling fraction is ~ 1/3 the gain [8], we designed the scraper to intercept 10% of the optical mode. This was done using the following analytical expressions [6]: The power transmitted for a gaussian beam of 1/e 2 radius ω through a circular aperture of radius a is given by:         − − = 2 2 2 exp 1 ω a Ptrans (1) In our case, we want P trans = 0.9, so a = 1.07ω. At the position of the scraper (z=15.4m), the mode size is given by: *shinn@jlab.org 222 M.D. Shinn et al. / Proceedings of the 2004 FEL Conference, 222-225 TUBOS03 Available online at http://www.JACoW.org